Hi folks!
Could any Quad 34 experts out there please tell me what the following components do, AND if they can be omitted/bypassed:
N1 - switch noise supression?
C77 and C78 (am aware of the orientation issue
)
IC23 (in PSU compartment) - is it worth changing this for something more modern, and if so.... any recommendations?
IC21 (PSU compartment again) - is it worth replacing this regulator? Again any tried and tested suggestions for a cleaner PSU?
Should add at this point all the usual Quad 34 mods (most are mentioned here on DIY Audio) have already been implimented to great effect. It's just the above I need to address for the last nth percent of improvement (well... a hopeful improvement ).
Many thanks.
- John
P.S. The circuit diagram may be found here: http://www.dc-daylight.ltd.uk/Valve-Audio-Interest/Schematics/QUAD-34-Schematic-Circuit.pdf
Could any Quad 34 experts out there please tell me what the following components do, AND if they can be omitted/bypassed:
N1 - switch noise supression?
C77 and C78 (am aware of the orientation issue
)IC23 (in PSU compartment) - is it worth changing this for something more modern, and if so.... any recommendations?
IC21 (PSU compartment again) - is it worth replacing this regulator? Again any tried and tested suggestions for a cleaner PSU?
Should add at this point all the usual Quad 34 mods (most are mentioned here on DIY Audio) have already been implimented to great effect. It's just the above I need to address for the last nth percent of improvement (well... a hopeful improvement
).Many thanks.
- John
P.S. The circuit diagram may be found here: http://www.dc-daylight.ltd.uk/Valve-Audio-Interest/Schematics/QUAD-34-Schematic-Circuit.pdf
Hi,
If it were me I would look at replacing all electrolytics on something this age. Use low E.S.R. 105 degree types, they are cheap enough, and make sure they are correct phsyical size.
IC 21 and 23, I don't think you will find any difference whatsoever swapping these. You need to use a scope and measure the rails to see how clean they are. A couple of inches away from the regs and it's the quality of the decoupling and the layout that matters most. N1 could not see it !
The OpAmps in the audio side of things, very definite yes here, I have my own favorites (OPA604 and OPA2604) they do sound good. Remember to check the current drawn by any OpAmps you swap, some types are very power hungry and it soon adds up.
Have fun
Karl
If it were me I would look at replacing all electrolytics on something this age. Use low E.S.R. 105 degree types, they are cheap enough, and make sure they are correct phsyical size.
IC 21 and 23, I don't think you will find any difference whatsoever swapping these. You need to use a scope and measure the rails to see how clean they are. A couple of inches away from the regs and it's the quality of the decoupling and the layout that matters most. N1 could not see it !
The OpAmps in the audio side of things, very definite yes here, I have my own favorites (OPA604 and OPA2604) they do sound good. Remember to check the current drawn by any OpAmps you swap, some types are very power hungry and it soon adds up.
Have fun
Karl
Thanks for your reply Carl.
Yep the electrolytics have been replaced by low ESR Panasonic FM units. Input cap for CD/Aux is also changed to Black Gate N type. new op-amps are OPA627's and OPA227s. They really improved the sound. Th ebiggest upgrade came from taking out ALL the poorly matched carbon film resistors, and replacing with hand-matched metal film types. I wouldn't want to do all this work ever again, but the results were well worth it. I also by-passed the balance control and this improved things also.
Cheers,
- John
Yep the electrolytics have been replaced by low ESR Panasonic FM units. Input cap for CD/Aux is also changed to Black Gate N type. new op-amps are OPA627's and OPA227s. They really improved the sound. Th ebiggest upgrade came from taking out ALL the poorly matched carbon film resistors, and replacing with hand-matched metal film types. I wouldn't want to do all this work ever again, but the results were well worth it. I also by-passed the balance control and this improved things also.
Cheers,
- John
Before the mods it was best described as 'warm and woolly' as the saying goes. Nice liquid midrange, but not much in the way of accurate bass or treble reproduction. Stock it really isn't much to write home about and I can see why some people dismiss it.
The mods have completely transformed it into a modern device which has bested my KingRex Pre-Amp and - to my ears at least - sounds better than a passive ALPS Blue on a box. But there's never any solid state harshness - it just sounds correct.
The mods have completely transformed it into a modern device which has bested my KingRex Pre-Amp and - to my ears at least - sounds better than a passive ALPS Blue on a box. But there's never any solid state harshness - it just sounds correct.
>>N1 - switch noise suppression?
Can you give a clue as to where this is on the schematic?
>>C77 and C78 (am aware of the orientation issue )
As suggested, replace with a good modern 105 degree low ESR electrolytic. Or use a RC network ( 100uF electrolytic in series with, say, 100 Ohms) paralleled with a film cap (say, 100nF polypropylene). I suggest these values as the RC network means that the electrolytic is 'working' in the bass region (below ~ 66 Hz) and the film cap covers the frequencies above that. Also the suggested values are likely to fit i the case - always a concern when modifying an existing piece of kit.
>>IC23 (in PSU compartment) - is it worth changing this for something more modern, and if so.... any recommendations?
This opamp sets the 'ground' reference because Quad was too cheap to install a transformer with multiple secondary windings. If you think about it, ALL signals must pass through this opamp. So it is as important any other in the preamp. Think of it like a dc servo. So an opamp with good dc precision is important (low Vos in particular - R123 1nd R124 are too low for bias currents to have any impact. Certainly for any reasonable opamp choice) - so select your favourite FET input opamp.
>>IC21 (PSU compartment again) - is it worth replacing this regulator? Again any tried and tested suggestions for a cleaner PSU?
The 7918 is certainly ancient but capable of excellent performance if implemented in an exemplary manner. You could replace this with an LM317 but this is NOT a drop in replacement as you would need to add extra feedback resistors and bypass caps. And the pinout is different which makes life awkward. Or you could one of the many 'super' regulator options that are available hereabouts - it depends on how much you wish to expend in terms of time and effort.
If you are looking for a less intrusive but still effective improvement, then add a series RC network ahead of the 7918 to improve its ripple rejection. C74 (1000uF) is already there so all you need to do is
1. Break the connection between the positive terminal of D34 and the positive terminal of C74 and insert a 22 Ohm resistor.
2. Break the connection between the negative terminal of D34 and the negative terminal of C74 and insert a 22 Ohm resistors.
This will increase the ripple rejection by > 20dB which is pretty much the performance gain you would get by replacing the 7918 with an LM317. I would mark up the schematic for you but the pdf is protected.
Hope this is of some help.
Can you give a clue as to where this is on the schematic?
>>C77 and C78 (am aware of the orientation issue )
As suggested, replace with a good modern 105 degree low ESR electrolytic. Or use a RC network ( 100uF electrolytic in series with, say, 100 Ohms) paralleled with a film cap (say, 100nF polypropylene). I suggest these values as the RC network means that the electrolytic is 'working' in the bass region (below ~ 66 Hz) and the film cap covers the frequencies above that. Also the suggested values are likely to fit i the case - always a concern when modifying an existing piece of kit.
>>IC23 (in PSU compartment) - is it worth changing this for something more modern, and if so.... any recommendations?
This opamp sets the 'ground' reference because Quad was too cheap to install a transformer with multiple secondary windings. If you think about it, ALL signals must pass through this opamp. So it is as important any other in the preamp. Think of it like a dc servo. So an opamp with good dc precision is important (low Vos in particular - R123 1nd R124 are too low for bias currents to have any impact. Certainly for any reasonable opamp choice) - so select your favourite FET input opamp.
>>IC21 (PSU compartment again) - is it worth replacing this regulator? Again any tried and tested suggestions for a cleaner PSU?
The 7918 is certainly ancient but capable of excellent performance if implemented in an exemplary manner. You could replace this with an LM317 but this is NOT a drop in replacement as you would need to add extra feedback resistors and bypass caps. And the pinout is different which makes life awkward. Or you could one of the many 'super' regulator options that are available hereabouts - it depends on how much you wish to expend in terms of time and effort.
If you are looking for a less intrusive but still effective improvement, then add a series RC network ahead of the 7918 to improve its ripple rejection. C74 (1000uF) is already there so all you need to do is
1. Break the connection between the positive terminal of D34 and the positive terminal of C74 and insert a 22 Ohm resistor.
2. Break the connection between the negative terminal of D34 and the negative terminal of C74 and insert a 22 Ohm resistors.
This will increase the ripple rejection by > 20dB which is pretty much the performance gain you would get by replacing the 7918 with an LM317. I would mark up the schematic for you but the pdf is protected.
Hope this is of some help.
Thanks for the tips guys
Hi Jan - Yep I've already bypassed the caps, and - like you - this really cleaned things up ALOT. Bypassing the balance control as well also helped a great deal. I never have had the need for balance controls so I don't miss it. Those tone controls can be handy on occasion though so I've left those in-circuit.
Vivavee, if I implement this RC network which side of it would goto the plus, and which to the minus in each case? I currently use 100uF Elna Silmic there. Maybe I should try bypassing that with a small value cap?
Hi Jan - Yep I've already bypassed the caps, and - like you - this really cleaned things up ALOT. Bypassing the balance control as well also helped a great deal. I never have had the need for balance controls so I don't miss it. Those tone controls can be handy on occasion though so I've left those in-circuit.
Vivavee, if I implement this RC network which side of it would goto the plus, and which to the minus in each case? I currently use 100uF Elna Silmic there. Maybe I should try bypassing that with a small value cap?
If you are referring to C77 and C78, then the resistor can go either side. It does not matter electrically, but there may be a better side in terms of physical placement on the PCB. I am at work at the moment and don't have access to the PCB. Just remember to preserve the polarity of the electrolytic caps.
Thanks so much for that diagram - that's great! I'll do that tomorrow
Would you be able to do a quick diagram of the C77/C78 mod as well to make sure I have it correct?
That 1000uF cap in the PSU has been replaced by a 2200uF Panasonic FC. I take it the PSU mod you show in the diagram us compatable with this?
Thanks again!
- John
P.S. As to N1 it doesn't appear to be any diagram, but it's in both Quad 34s I own. It's between neutral and live on the input, and is a combined Rifa 0.047uF capacitor/ 100ohm resistor in series, in potting compound. I think it's to lesson on/off pops. Removing it didn't seem to make a difference.
Would you be able to do a quick diagram of the C77/C78 mod as well to make sure I have it correct?
That 1000uF cap in the PSU has been replaced by a 2200uF Panasonic FC. I take it the PSU mod you show in the diagram us compatable with this?
Thanks again!
- John
P.S. As to N1 it doesn't appear to be any diagram, but it's in both Quad 34s I own. It's between neutral and live on the input, and is a combined Rifa 0.047uF capacitor/ 100ohm resistor in series, in potting compound. I think it's to lesson on/off pops. Removing it didn't seem to make a difference.
Vivavee - thanks for your mods. Implemented the PSU mod today and it does appear to have cleaned up the sound a tad. Changing to Shottky rectifiers helped as well.
I haven't attempted the C77 / C78 mods yet as I'm happy with the sound at the moment (using 100uF Elna Silmics here), but once the itch to mod comes on again I'll certainly try it
Cheers,
- John
I haven't attempted the C77 / C78 mods yet as I'm happy with the sound at the moment (using 100uF Elna Silmics here), but once the itch to mod comes on again I'll certainly try it
Cheers,
- John
I really do think it's a fantastic pre-amp now, and - believe me - I know why people used to mock its sound.
I sounds like a passive pre now..... with balls
I'd say that hand-matching resistors between channels, and upgrading the caps and switching bank ICs probably had the most effect, then the op-amps, then the PSU.
I sounds like a passive pre now..... with balls
I'd say that hand-matching resistors between channels, and upgrading the caps and switching bank ICs probably had the most effect, then the op-amps, then the PSU.
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